This invention relates to coolant flow control in fluid-cooled articles and, more particularly, to the control of coolant in film-cooled articles such as gas turbine engine plug nozzles in which the area of the coolant slot is affected by the relative displacement of the various nozzle components due to aerodynamic, thermodynamic and mechanical loadings.
The high temperatures generated in current advanced technology gas turbine engines require that various components be cooled to prevent thermal erosion and fatigue. The cooling problem is particularly challenging in the combustor, turbine and nozzle portions of the engine where temperatures are most severe. While improved high-temperature materials have been developed which can better withstand this environment (turbine inlet temperatures in excess of 2000.degree. F), they must invariably be augmented by some type of fluid-cooling scheme.
Basically, three types of fluid cooling have been developed which are used either singly or in combination depending upon the temperatures encountered and their ease of incorporation. These three types of cooling are commonly referred to as convection, impingement and film cooling, and their use is well understood by those skilled in the art. The present invention is directed to those components which are film cooled by a layer of cooling air which is injected between the high temperature gases and the hot gas side of a flow path defining wall. Generally, the layer of cooling air is formed by directing airflow from a cooling plenum located on the side of the wall opposite the hot gas flow through a series of apertures within the wall. The apertures can take the form of holes from which the air is ejected normal, or near normal, to the hot gas stream. Alteratively, overlapping, telescoping sections of the wall can be arranged to form slots or gaps extending substantially parallel to the wall and from which the cooling air is ejected as a film with less turbulence and mixing than with the more perpendicular holes. While this latter approach to the problem has several aerodynamic advantages, at least one significant drawback has existed. Namely, where two overlapping members are separated by the gap for spreading a cooling air film over the overlapped member, relative thermal displacements of the two members will vary the size of the gap and thus modulate the cooling flow rate, often in an undesirable manner. Additionally, the size of the gap will be affected by aerodynamic and mechanical loadings. This phenomenon is of particular concern in lightweight, sheet metal gas turbine engine exhaust nozzles which experience wide gradients in temperature as the engine power level is modulated.
Various prior approaches to the problem include the use of structural elements to maintain a predetermined gap between the interfacing members through which the controlled amounts of cooling airflow pass. However, these structural elements are subject to thermally induced fatigue due to the cyclic relative displacement of the interfacing members. In another instance, the slot cooling concept was abandoned in favor of several thousand small-diameter, precision-drilled holes which, in essence, provided a porous structure through which the coolant could be aspirated as a film. However, since drilling cooling holes is notoriously expensive, the cooling slot approach is favored and it is desirable to find a device which can meter the flow through the gap between two overlapping members irrespective of the size of the gap as it is affected by the relative displacement of the members.